The resounding image in the poem is the joy of nature. The images of the grasshopper and cricket's joyful noises, the warmth of the sun and the cool relief of the shade, and the freshly mown mead all add to a feeling of happiness and celebration. Keats focuses on the cricket and the grasshopper as the celebrators of nature. They are nature's soundtrack, and her trumpeters.
This poem was written in competition with Keats's friend Leigh Hunt. They made up the topic and each had a go at a sonnet, which has a set fourteen line structure usually divided into an octet (first 8 lines) and a sextet (last six lines). Keats leads off with the assertion of the first line and then gives a supporting example from the summer followed by the winter. Lines 2 through 8 are a sizzling summer day -- even the birds are bothered -- but he shows a lively grasshopper bouncing around happily in the shade of the grass. The images make you really feel the heat and sense the cool of the shade. In the last lines he switches to the opposite -- winter -- it's cold and you're glad to have the heat of the fire. And a cricket is also there, sheltering near the fire, singing away. In the last line he masterfully ties it together by having a drowsy person near the fire think it's actually summer again, coming back to the beginning in a kind of circle. The imagery and the words are chosen to be "sensual" -- so you actually can FEEL what he's talking about -- Keats's poetry is like that. Help?